Nitride based blue laser diodes are being developed for printing and optical data storage applications. The first AlGalnN laser diodes were broad area lasers providing no control over the laser diode's various spatial modes. Most applications, however, require the laser diode to operate in a single spatial mode. One way of achieving single spatial mode operation for AlGalN blue laser diodes is to use a ridge waveguide structure to define a lateral waveguide as described in “Ridge-geometry lnGaN multi-quantum-well-structure laser diodes” by S. Nakamura et al., in Applied Physics Letters 69 (10), pp. 1477-1479 which is hereby incorporated by reference in its entirety. While a ridge waveguide provides for single spatial mode emission in blue lasers, the waveguiding provided is relatively weak. The lateral refractive index step is small and is influenced by heating and carrier injection. Additionally, there are fabrication difficulties because the ridge must be etched to extend sufficiently close to the laser active region without the ability to use an etch stop to prevent material damage to the laser active region since chemical etching is not applicable to GaN materials.
To provide stronger mode stability and low threshold current operation, more strongly index-guided diode lasers are required such as those having buried heterostructures that are typically used for InGaAsP fiber optic-communication lasers, or the impurity-induced-layer-disordered waveguide structures used for high-power single-mode AlGaAs laser diodes. Additionally, the use of a buried heterostructure avoids certain fabrication difficulties.